Balanced and
Unbalanced Systems

To understand how a balun operates and why a balun is
needed, we first must understand balance. Most often balance is described only
by current in each conductor of a transmission line. That
can mislead or confuse us. There have been many articles incorrectly describing
measurement of balance. Test equipment improperly measuring balance has been
manufactured and sold.

Perfectly balanced lines and perfectly unbalanced
lines all have equal and opposite currents entering and leaving the conductors
at each end, as can lines that are out of balance. All properly operating
two-conductor transmission lines, coaxial or parallel wire, carry equal and
exactly opposite phase currents in the two conductors. Non-radiating coaxial
lines and parallel lines (twin lead or ladder line) both have exactly equal and
opposite flowing currents entering or leaving each conductor at any given end of
the transmission line.

The only thing determining the gender of balance
is the electric field to space surround the line, or the conductor voltages to
"ground" or space around the line.

We can establish these rules for properly operating
transmission lines:

all lines have equal and opposite currents in
each conductor

perfectly unbalanced lines have zero electrical
field (voltage) outside the line to space or "ground"

perfectly balanced lines have equal and opposite
electric fields to space around the line, or to "ground"

Currents flowing without equal, very closely space returning currents,
are called common mode currents. Common
mode currents cause coupling and radiation. In a dipole
antenna, or any antenna for that matter, common mode currents in the antenna
element(s) are entirely responsible for radiation. Inside the Ham
shack or along the antenna feed line,
common mode currents are responsible for unwanted noise ingress, RFI, RF burns,
and a host of other maladies. Common mode currents effectively bring the radiating
part of the antenna system down along the feed line or the antenna's metallic
supporting structure. Common mode currents can extend all the way to the desk
and station equipment, and even out through power line connections.

We generally do not want our feedlines to act like
antennas, and we certainly don't want to listen to the electrical noises in our
house wiring. The best idea is generally to keep common mode currents out of the
feedlines and control cables. Common mode currents should thus be "dealt with" at the source of those currents,
or as close to that as is most effective. This is where the the problem actually
is.

Some sites claim impedances in the thousands of ohms are necessary to
reasonably or properly eliminate
common mode. Such claims are
truly absurd. Even in critical systems, the system has a major
design or layout problem if it requires more than a few hundred ohms common mode suppression
impedance.

In general, a properly constructed layout and proper
antenna with good connections and cabling will not benefit from suppression
inside the building. Even a severely compromised layout, such as with the
antenna close to RF sensitive devices and/or equipment, the equipment should still
follow good basic layout and wiring principles.

There is a side benefit
to a good layout. The
very things reducing common mode issues also reduce lighting damage
susceptibility.
Lightning protection and common mode RF immunity go hand-in-hand together.

Once
the station or equipment is properly installed, even relatively small amounts of
additional common mode impedance will offer a significant reduction in common
mode. A proper system will not need more than a few dozen ohms or hundred ohms
additional isolation. A poor layout might not be improved with nearly infinite
isolating impedance. One thing is universally sure, if a system needs more than
a few hundred ohms CM suppression the system has a major layout or wiring
problem. It is best to fix that problem before adding
chokes, and worrying about obtaining choke impedances
impossible to achieve and maintain in a real-world system is pointless.

Placing things in perspective, on 40 meters just 9 pF of stray capacitance is
2500 ohms. Do we really honestly think, with even a few feet of wire or a
system device the size of a small mobile radio cabinet, we have could have less
than 25-100 pF of stray coupling to other things? With that in mind, would
something like 1000 ohms even make any sense at all? Don't waste time chasing
rainbows. Spend your time and money actually doing something useful.
Anyone can make something "work" or produce a large number in an uncluttered
controlled test bench. The real world and how things work in the real world is
what matters. If you need more than several hundred ohms suppression,
something is seriously wrong with the equipment, antenna, or feed system design
or layout.

Proper Layout

All proper installation should have a common point where cables enter for all
desk functions, including power, Internet, control, and RF cables.

A common mode choke alters the common mode impedance of the system. The
isolation added to any system, just like the impedance required, is not predictable.
Isolation can only be measured with a great
deal of work. We can say one thing with some certainty, large values of common
mode impedance are rarely necessary.

Attenuation with the addition of a choke is a function of
the common mode source impedance, the common mode line impedance on both sides
of the choke, and the impedance of the line's ground at the equipment side. This
is not a simple system. Common mode source and termination impedances are
virtually never anywhere around
the 50-ohms a typical S21 or S12 measurements are made with. Because of this,.
measured or advertised attenuations are meaningless numbers and any blanket prediction of system
choke impedance needs are pretty much useless. All that can be said is it is
best to use a dissipative impedance (Q<1) rather than reactive (Q>>1) in RFI
suppressor systems.

In "A" below, a transmitter drives the coaxial feedline in
differential (push-pull). The center conductor in this example A's 50-watt
transmitter, assuming matched lines, will be at 50 volts. The shield is ideally
at 0 volts to earth, being a commonly grounded point with earth. The current are
equal and opposite in properly operating balanced and unbalanced lines, but in
normal operation the shield being at zero volts to "ground" (or to the
chassis) defines the system as being a perfectly unbalanced system.

If the system were perfectly balanced transmission line currents would still
be equal and opposite. The transmitter voltages from each feeder conductor to
chassis or ground, however, would be equal and opposite in a balanced line
system. The line would also have two parallel conductors equally coupled to
earth or ground through space.

Coaxial line behavior, from skin effect and mutual coupling from the
shield's inner wall to the center conductor, pulls all of the transmitter's
differential current to the area inside shield wall. The voltage appearing at the antenna
drives the common mode impedances of both halves of a "balanced" antenna,
pushing one side of the antenna against the other antenna half.

A
halfwave dipole, depending on construction, height, and surroundings, has
essentially equal resistances. We'll use the values shown as a discussion point.
If antenna current was one ampere and no common mode flowed on the coax, each
half of the antenna would have 25 volts to earth at the feedpoint. The
voltage developed across Rant2 causes the common mode problems, because that
voltage also excites the shield on the shield outside.

Allowing that voltage to "float", in this example to 25V, is the same as
preventing unwanted outside shield current from developing.

Rant2
was near zero ohms, it would be a unbalanced antenna with a very good ground.
The better the ground, the lower "Rant2" below, the less voltage is in series with the
resistance driving the shield (B).

This system is complicated by antenna characteristics and other things. The
antenna system common mode impedance and grounding determines the source
impedance and voltage driving the shield with common mode.
The system is further complicated in that cable1's length and surge impedance
modifies the voltage and impedance at the common mode choke. In its most simplified
form, the common mode choke system looks like a pi-attenuator. The common mode
source impedance at the antenna, as modified by the cable common mode surge impedance
and
shunting leakage and ground paths modifies voltage and impedance driving the
CMC (common mode choke).

Similarly, common mode impedance into the station
equipment, likely
a pretty low impedance, determines the station side load on the pi attenuator
formed with CMC.

The attenuation and required CMC impedance quite obviously can be all over the
place. While none of us can predict the real attenuation provided by a given CMC
or predict how much CMC impedance is "enough" and how much is a waste
of effort, we can make a pretty reasonable generalization. It is very safe to
say any system requiring CMC impedances beyond low hundred's of ohms needs
layout, design, or re-wiring help more than some extraordinary impedance.

My station, where lowest possible noise floor is paramount, has never
required more than a 50-100 ohms of CMC to mitigate all traces of problems. I've
actually found grounding and good connections to be far more productive and
reliable than dependence on a higher CMC impedances. Once I get into
the dozens or hundreds of ohms without complete mitigation of noise, I know I
have a major problem with shield integrity. I look for the real problem.

Common Mode Currents, Baluns, and CM Chokes

To understand how a balun operates and why a balun is
needed, we must understand balance. We tend to think of balance only in the
amount of current in each conductor of a transmission line, but that thinking
can mislead or confuse us. Perfectly balanced lines and perfectly unbalanced
lines alike have equal and opposite currents entering and leaving the conductors
at each end!

Coaxial cables with shields more than several skin depths
thick always carry equal and opposite flowing currents on the inside of their
shields and their center conductors. Current direction and current ratio between
the center conductor and inside of the shield in a non-radiating coaxial line is
no different than currents in each conductor of a perfectly balanced ladder
line. In both unbalanced coaxial lines and balanced lines, the two conductors
making up the line carry equal and opposite flowing currents.

When currents flow without close-by opposing currents, we
call the unopposed portion of current common mode current. Common
mode currents promote or encourage external coupling and radiation. In a dipole
antenna, or any antenna for that matter, common mode currents in the antenna
element are responsible for radiation. In the hamshack or along a feed line,
common mode current is responsible for unwanted noise ingress, RFI, RF burns,
and a host of other maladies. Common mode currents, in effect, bring the
radiating system into the feed line or station equipment.

Common-mode currents, or currents flowing in the same
direction, cannot exist inside a coaxial cable at any frequency
where the shield is several skin depths thick. Shield skin depth serves to
isolate the inside of the shield from the outer wall of the shield. Common mode
(same direction) currents can only flow on the outside of the coaxial cable
shield. Differential mode currents, or normal transmission line currents, flow
on the inner surface of the shield wall. Currents entering and leaving the
shield and center conductor at each end of a coaxial line must be equal and
opposite or the cable will radiate. If a coaxial line is not radiating, currents
in the shield and center conductor are exactly balanced and opposite flowing.
Both types of transmission lines, balanced and unbalanced, will have equal and
opposite currents entering and leaving each conductor when they have minimal
radiation.

What then defines an unbalanced line, source, or load? The
answer lies in the voltage or electrical potential between line conductors and
the environment around the line. In the ideal balanced line, the electric
potential of each conductor is equal and opposite in relationship to the
environment surrounding the line including the chassis or cabinets of our
equipment. In the ideal coaxial line, the outside of the shield has no
electrical potential difference to the environment around the line, including
the chassis or cabinets of our equipment. The shield of our coaxial cables, as
we commonly accept and understand, is at ground potential. We say the shield is
“grounded”.

With real-world antennas, the coaxial shield connection
point almost never has zero electrical potential to the environment around the
shield or points further along the cable’s length. Being a less-than-ideal
zero-voltage termination, shields almost always have common mode current, even
if a small percentage of differential (normal transmission line mode) current.
For example, the four radials of a groundplane antenna, no matter how configured
or tuned, are never truly at the same electrical potential as the environment
around the antenna or shield potential further down the feed line. Experimenting
with a groundplane antenna, we find the feedpoint is mostly but not perfectly
unbalanced. The shield is not connected to an electrically zero point.
Significant current can and often does excite the outside of the shield on a
groundplane antenna, with outside shield current 20% or more of antenna base
current under some feed line grounding and lengths! We consider the groundplane
antenna “unbalanced” and it is definitely not balanced, but it is not perfectly
unbalanced.

A coax fed dipole, or a vertical with a single radial, is
much worse for voltage balance. Because both antenna halves have finite and
nearly equal common mode impedances, both sides of the feedpoint want to have
nearly equal voltages between themselves and the environment around the
feedpoint. If we could magically make a perfect single point ground appear at
the feedpoint, both legs of these antennas would have very similar voltages to
that reference point. Of course we can’t make that perfect reference point
appear, but the feed line brings a “ground” or reference connection to the
feedpoint. Third path impedance (the unwanted common mode path impedance) varies
with feed line length and routing, the environment the feed line routes through,
and how that feed line is grounded. While it can affect SWR and the current
flowing in that path be affected by SWR, high SWR does not cause and low SWR
does not prevent unwanted common mode current or RF in the shack.

Most antennas are neither perfectly balanced nor perfectly
unbalanced. Most antennas are in a nether-world someplace between perfectly
balanced and perfectly unbalanced. This is why a current balun, a device that
floats each balanced terminal to the voltage necessary to drive balanced
currents into the load, is such a desirable type of transmission line to antenna
interface.

Feedlines and Balance

Traditional
two-conductor
feedlines or
transmission lines
should have very
little, if any,
radiation. Typical coaxial
lines shouldn't have noticeable
unwanted signals or noises
leaking into the
cable, and there
shouldn't be
noticeable radiation
out of cables.
This is true at all radio frequencies from the AM broadcast band upward through
UHF, and applies to any cable with a reasonably thick single or double shield. Even
cheap 80% coverage shields are adequate throughout HF in all but the most
critical applications.

With unshielded
two-conductor
balanced lines,
some
small amount of
radiation from the line, or
leakage into the
line, exists. The leakage
amount should be
very low in properly installed
two-wire balanced
lines. Problems can occur with an unwanted EMI
source, or sensitive systems,
close to the
feed line. Problems with unwanted signal pickup or radiation can also occur with
very long open wire or unshielded balanced lines.

Ladder lines and
unshielded balanced RF lines should be isolated from other objects. Ideally, the
only air should be allowed within several conductor spacings of balanced
or unshielded lines. Significant levels of electric and
magnetic induction
fields surround the
line for distances of several transmission line conductor spacings. Line radiation
(electromagnetic radiation is a different mechanism) also extends out in a line
through the conductors. Unwanted radiation
primarily occurs
in directions aligned with the plane of the line conductors, nulling at right
angles to that plane.

Even with perfect balance in a two-conductor unshielded line, some radiation occurs. The
small spatial
separation prevents perfect cancellation of far field radiation. The
conductors carrying
out-of-phase
currents are not
occupying the same
physical space, causing a very small spatial phase delay. This means in two
directions radiation fields are not precisely 180 degrees out-of-phase. The
amount of phase error, and thus the level of radiation, is a function of
conductor spacing in wavelengths and the direction from the line. This
effect is minimized
by twisting the feed line at small
fractions of a
wavelength.

Radiation from a
perfectly terminated
six-inch spaced
50-foot long
two-wire
transmission line on
80 meters.

Radiation of the
same line at 30 MHz
is 24 dB stronger.
This is because the
conductor spacing in
wavelengths is
wider.

In order to be
balanced, a balanced
transmission line
must have both equal
and opposite
voltages at any
particular point
along the line as
well as equal and
exactly opposite
currents at any
particular point
along the line. If
the voltage
is not equal and
opposite, current
cannot remain equal
and opposite along
the balanced
transmission line.
This will result in
a very large
increase in feed line
radiation because
the imperfection
causes common mode
currents.

In order to be
unbalanced, an
unbalanced
transmission line
must have equal and
exactly opposite
currents entering
and leaving at every
point along the
line. The voltage
gradient laterally
along the outside of the transmission line has
to be zero. If
either the lateral
voltage gradient is
not zero, or currents entering the line are not equal and opposite on the shield
and center, current will not remain zero on the outside of the shield. This will
result in common-mode
shield currents and feed line radiation.

To
avoid feed line
radiation every
balanced to
unbalanced
transition has to be
properly treated for
level and phase of
voltage and
current.

To be properly balanced, the following must occur:

Voltages from 1 to A, and from 2 to A, must be equal and
opposite

Currents into 1 and 2, at the source, must be equal and
opposite

Voltages from 1 to C, and from 2 to C, at the load must
be equal and opposite

Currents out of 1 and 2, at the load, must be equal and
opposite

Voltages all along the line, at any
point, to B must be equal and opposite

Common Mode
Excitation

Common-mode
current is current
that is not opposed
or counteracted by
an equal and
opposite phase
current flowing
at every point along
the line in closely-spaced conductor or
conductors, and the
outside of the
shield has current
flowing in a coaxial
line.

Any
transmission line
becomes at least
partly, a
radiating conductor
if we make a poor
balanced to
unbalanced
transition.
This can be useful
when we wish to use
a feed line as an
antenna or as a
conventional
conductor, but it
can be detrimental
to a system if we do
not want radiation
or reception by our
feed lines. When we
excite a cable as
shown below, we have common
mode current:

The common mode source is
end-to-end on one or
more conductors of
twinlead

The common mode source is
end-to-end on one or
more conductors in a
twisted pair of
wires

When we excite
a transmission line
as shown below we create
common-mode current:

The shielded coaxial
cable (top) and the
parallel conductor cable
(bottom) in this
diagram radiates
just like a single
wire would do.
Objects surrounding the line,
like dielectrics or
other conductors,
couple to or
interact with these lines
when they are fed or
excited this way.
For example, adding a ferrite
sleeve over the
lines will add loss
and/or make the lines
behave differently.
The impedance of the
system will change,
and if we are
watching system SWR
the SWR will change.

This is true even
when currents are
equal in the two
conductors, and can
even be true when
currents are equal
and opposite at one point in the system, so long
as the line is
excited this way.

The key to having
a line behave like a
transmission line is
feeding it
differentially
(across the two
conductors) at
one end, having a
load that maintains
the differential
excitation, and not
applying a voltage
or a potential
difference
across the length of
one or both
conductors.

This is a
transmission line as
we generally know
it, and as dozens of
reputable
engineering
textbooks define it:

This is
differential mode,
or TEM mode. This is
the normally desired
excitation mode when
a two-wire line
behaves like a
normal non-radiating
line that transfers
energy from one
point to another.

The above
configuration shows
a direct wire
connection from
source to load. It
transfers
voltage, current, or
impedance directly
along the conductors.

A
1/8th wave long
(35 feet in this
case) coaxial feed line
to the ground point
on a dipole often does not need a
balun! Here are
feeder common mode
currents for this
case:

location

amperes

Left leg

0.953

source

1.000

Right leg

1.001

1 ft

0.052

2

0.054

3

0.056

4

0.057

5

0.058

6

0.060

7

0.061

8

0.062

9

0.064

10

0.065

11

0.066

12

0.067

13

0.068

14

0.069

15

0.070

16

0.071

17

0.072

18

0.073

19

0.073

20

0.074

21

0.075

22

0.075

23

0.076

24

0.077

25

0.077

26

0.078

27

0.078

28

0.078

29

0.079

30

0.079

31

0.079

32

0.079

33

0.079

34

0.079

35 ft

0.079

Maximum feeder
common mode is only
.079 amperes (out of a 1 ampere source current) with
very good antenna
current balance. This is without any balun!

The same dipole 1/4 wave high:

location

amperes

left side

0.996

source

1.000

right side

1.002

shield at dipole

0.018

4

0.015

6

0.012

8

0.012

10

0.012

12

0.014

14

0.017

16

0.021

18

0.024

20

0.028

22

0.032

24

0.035

26

0.039

28

0.042

30

0.046

32

0.049

34

0.052

36

0.055

38

0.058

40

0.061

42

0.064

44

0.067

46

0.069

48

0.071

50

0.073

52

0.075

54

0.077

56

0.079

58

0.080

60

0.081

62

0.082

64

0.083

66

0.084

68

0.084

ground

0.084

Common mode
currents are also low
with a 1/4 wave
feeder. In many
or most cases of dipole height between 1/8th and just over 1/4 wavelength, a balun
is NOT necessary provided the feed line drops straight down through open air to
ground, and the feed line is grounded when it reaches the earth.